Subject: [Tweeters] Re: Ten Thousand Crows
Date: Dec 21 15:50:11 2012
From: Kevin Purcell - kevinpurcell at pobox.com


On Dec 20, 2012, at 10:57 PM, Lorindon wrote:

> As a Bothell resident, living on Ross Road just north and west of the campus, we enjoy the crow invasion every dusk. They consume our neighborhood, and then when they're ready they all fly off.

Sounds like a pre-night-roost assembly. I know that Rooks and Jackdaws use this approach before night roosting in the UK: they'll assemble at multiple different location before heading to a night roost en masse.

It seems American Crows have the same behavior. :-)

A similar technique is used in the morning when a night roost dissipates groups of crows follow each other to large daytime assembly points. Volunteer Park in my local patch of Seattle comes to mind. Water for washing seems to be an important requirement. I even saw a decent sized daytime assembly in the Magnuson Park car park last time I was there that seemed to be focused on puddles in the parking lot.

It's suspected that information transfer takes place in these roosts and assemblies (the information center hypothesis).

It's been shown for a large raven roost at Newborough on Anglesey in North Wales that birds are recruited to eat carrion (laced with indigestible pellets so traces from regurgitated pellets can be found distributed around the roost). A bird that didn't eat well one day can follow birds that roost close to it back to their source of food and can help the recruiter eat the carrion. Game theory seems to suggest this "gang" recruitment for large carrion items is evolutionary stable strategy.

The curious might find this paper an interesting read: http://www.avibirds.com/pdf/r/raaf8.pdf

On Dec 20, 2012, at 11:12 PM, jeff gibson wrote:

> Moving right along, one thing I find really interesting in the movement of crows is the different modes of flight I've been noticing. When people talk about "River of Crows" is does bring to mind Fluvial Geomorphology, the study of flow and resulting landforms made by rivers and streams. Rivers come in different forms you know. With crows coming into Bothell I've noticed a broad flow of equally spaced birds, at moderate height. Then I've noticed what I might call a "braided creek" of crows- very fast moving dense 'ropes' of crows merging together at times. They look like crows shot out of a hose, usually flying low. Whats up with that? Looks like fun is all I know.

You might, perhaps, consider the roost and assembly points as lakes or sink holes when thinking this way.

Roosts can be quite complex in structure (having sub roosts that are used less frequently but the birds keep coming back).
--
Kevin Purcell (Capitol Hill, Seattle, WA)
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